Means for obtaining a uniform evaporated deposit



B. H. VINE 2,81 1

MEANS FOR OBTAINING A UNIFORM EVAPORATED DEPOSIT Jan. 7, 1958 Fil ed Feb. 18, 1955 R W my m MEANS FOR OBTAINING A UNIFORM EVAPORATED DEPOSIT Benjamin H. Vine, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application February 18, 1955, Serial No. 489,050 7 Claims. (Cl. 118-49) This invention relates to television pickup, or camera tubes and particularly to a method of and means for obtaining a uniform photosensitive layer in television pickup tubes.

The type of television pickup, or camera, tube with which this invention is particularly concerned is a tube that is known commercially as a vidicon. This type of tube is manufactured in two sizes, i. e. tubes having an envelope one inch in diameter and tubes having an envelope two inches in diameter. Tubes of this type briefly comprise an electron gun in one end of an evacuated envelope. In the other end of the envelope is a target which includes a transparent conductive signal electrode substantially covered by a photoconductive layer.

During the manufacturing processes of such pickup tubes, there has been a problem of obtaining a uniform photoconductive layer. More specifically the problem has been that of obtaining a porous, uniform photoconductive layer of antimony tri-sulphide, as distinguished from the non-uniform thicknesses of photoconductive material found at times in the prior .art.

One solution to the above mentioned problem which is reasonably satisfactory in tubes 1 inch in diameter, is to deposit the photoconductive layer by evaporation, with the evaporator crucible about of an inch from the signal electrode, in an inert gas atmosphere at a pressure of the order of 1 mm. of mercury. However, when an attempt is made to utilize this solution with the evaporator crucible at a distance greater than of an inch from the surface onto which the photoconductive material is to be deposited, which is desired when utilizing certain manufacturing techniques, the evaporated material does not deposit uniformly in a radial direction. Also, this solution does not provide satisfactory results in tubes of larger diameter. For example, when attempting to use this solution in a 2 inch tube, a concentric ring deposit of a diameter of about half that of the tube is usually formed. The reasons for this concentric ring deposit photoconductive material are not clearly understood.

It is therefore a principal object of this invention to provide a new and improved means for, and method of, obtaining a uniform photoconductive layer in a pickup tube.

The above object, as well as various other aspects and advantages, are accomplished in accordance with this invention by providing an evaporator shield in a pickup tube which includes an apertured tubular mesh member. In an embodiment of the invention the evaporator shield may remain in the tube and be utilized as a grid electrode in the completed tube. An improved method of manufacture in accordance with this invention is to deposit a porous layer of photoconductive material while shielding the evaporator crucible with the evaporator shield.

The novel features which are believed to be characteristic of this invention are set forth in the appended claims. The invention itself will best be understood by reference to the following specification when read in conice nection with the accompanying single sheet of drawings, wherein:

Figure 1 is a transverse sectional view of a pickup tube in accordance with an embodiment of this invention;

Figure 2 is a schematic representation of a means for inserting and withdrawing an evaporator crucible into and out of the tube shown in Figure 1; and

Figure 3 is a fragmentary transverse sectional view of an embodiment of this invention.

Referring specifically now to Figure 1 there is shown a transverse sectional view of a pickup tube 10 in the process of having a photoconductive layer deposited therein. The pickup tube 10 comprises a sealed envelope 11 having an annular conductive ring 12 sealed to one end thereof. Sealed to the other face of the annular conductive ring 12 is a transparent face plate 14. The envelope 11 may be glass while the conductive ring 12 may be of some material which will readily seal to glass such as a cobalt nickel-iron alloy. The face plate 14 is preferably of some type of highly transparent, optically clear glass.

Deposited on the inner surface of the face plate 14, and in contact with the conductive ring 12, is a transparent conductive layer 16 which may be of a material such as tin oxide. On the exposed surface of the transparent conductive layer 16 is a photo-conductive layer 18 which may be a material such as antimony trisulphide. Specific details concerning the means for depositing the photoconductive layer 18 onto the transparent conductive layer 16, will be explained hereinafter.

Supported in the other end of the envelope 11 is an electron gun 20 which comprises the usual thermionic cathode 22, a control electrode 24 and one or more accelerating electrodes 26. Since the structure of gun 20 is well known, and also since it is not a part of this invention, further description thereof is not deemed necessary. Lead-in wires 27 for the electron gun 20 extend through the end of envelope 11 in the usual manner. Surrounding the envelope 11 there is provided beam controlling means for controlling an electron beam developed by gun 20. The beam controlling means forms no part of this invention but is shown merely to show the position of the means with respect to this invention. The beam controlling means focusses the electron beam, and scans the beam over the photoconductor 18 to form a raster, and may include a focus coil 28, a deflection yoke 30, and an alignment coil 32.

Extending from closely adjacent to the photoconductive layer 18 to beyond one end of the electron gun 20 is an evaporator shield 34. In accordance with this embodiment of the invention, the evaporator shield 34 comprises a hollow tubular member 36, a hollow tubular mesh screen member 38, and a flat mesh screen 40. The flat mesh screen 40 is connected to the hollow tubular mesh screen member 32 by being spot Welded to a ring 41 which is welded to tubular screen member 38. Tubular screen member 38 may be spot welded to hollow tubular member 36. Other conventional means of joining metallic members may also be utilized. The evaporator shield is centrally supported Within envelope 11 by means of spacer clips 42. The cylinder 36 may be made of a nickel-chrome alloy, while the mesh member 38 may be made of stainless steel which may be gold plated to improve corrosion resistance. The number of wires per inch in mesh member 38 is not critical and mesh screens having 30 to wires per inch have been successfully utilized. The flat mesh member 40 may also be made of stainless steel and preferably has a minimum of approximately 500 wires per inch so that mesh member 40 may function as an electrode in the completed tube. Other types of apertured tubular members having one rator shield 36 when inserted within envelopell'i The crucible 48 may bemade-of a hardglass -and in actual practice may b'eapproximately /3 inch in diameterand inch deep. The heatingjelement 50'imay bea coiiwire which may be energized by several-iconventioualmeans one of'which will bc explained in connection with Figure.2.'

Referring now to'Figure 2*1thereiis'shown onemethod of, and means for, inserting andwithdrawing,thecrucible 48' into andiout of an envelope 11f ,The. envelope llgis connected to a hollow" glass manifold tube 51which encloses theevaporatorprior: to its insertion into theenvelope 11. The'manifoldtube.51, which includes an exhaust tubulation 53 for connecting to' an evacuating pump (not shown), is sealedto the exhaust'tubulation 44 of envelope 11. Withthe gun assembly 20 within the envelope, the manifoldSl is connected'at an off center location on envelope 11. The evaporator assembly may be moved into; andout.of,envelope 11" by means of a magnet 57acting, on an iron armature 58' attached to the evaporator;

One means for heating the. heating'element 50', which surrounds crucible 48, comprisesutili'zing a portion of the s'upport'wires 46'within tube 51' as a secondary'ofa transformer having a primary 56 outside of' manifold tube 51: The primary 56 is connected'to a source of electricalenergy (notshown): Of'course, other means (not shown) for energizing the heater element 50' are within the contemplation of'this invention such-as contacts which engage fixed lead-in wires through the walls of the glass manifold tube 51.

To deposit a surface of photoconductive material '18, the tubulation 44 is sealed to the manifold 51, the env'elope His evacuated and filled withan inert-atmosphere, e. g. argon at a pressure of approximately 1 mm; of mercury, and the evaporator assembly 46 is inserted into the envelope and with-the evaporator crucible 48 approximately 1% inches from the transparent conductive coating 16. When these steps have been accomplished, the evaporator crucible 48 is heated to-a temperature abve-the melting point of the antimony tri-sulphide, i. e. above 550 C. In actual practice, the evaporator crucible 48'is probably heated to a temperature of-600 to 700 C. Due to the presence of the tubular mesh screeen 38, the deposit of the material 18 is substantially uniform. The reasons for the uniform deposit are not clearly understood. Perhaps the heat of the evaporator causes convection currents in-the gas atmosphere of the prior art methods'which carry the evaporated particles into anon-uniform pattern. Assuming that-convection currents are the cause of the non-uniform deposits in devices prior to this invention, the mesh screen of this invention breaks up the convection-currents resulting in a uniform deposit.

Another possibility for the cause of'the unevenness in previous devices, for which there is some evidence, is that particles of photoconductive material radiating from the evaporator strike a solid wall cylinder and are specularly reflected to form the observed patterns on the faceplate. If this is thecause of the-non-uniform deposits in previous practices the present invention provides a uniform deposit by deflecting the evaporated particles in many directions thus providing a-uniform deposit of any deflected particles: Althoughthereasons as to why auniforrn deposit is'obtained by following the teachings of "this' invention arenot clearly understood, it has been The evaporator comprises a crucible" 48 4 proved by many experiments that the results obtained by-practi'cing this invention provide a uniform deposit of photoconductive material.

After the step of depositing the photoconductive material 18 by evaporation, the evaporator crucible 48 is removed from the envelope 11 by being drawn back into manifold 51, by means of magnet 57. The envelope 11 is then evacuated, and tubulation 44 is sealed 0E. The approximate-"sizesof the elements, to permit this insertion and withdrawing of evaporator crucible 48 are that the evaporator crucible is; approximately inch in diameter, the gunuis approximately /fi inch in diameter whiletheenvelope v11 is approximately 2 inches indiameter;

In this embodiment of the; invention the evaporator shield 38 is left within the envelope 11 to function as the final accelerating electrode for an electron beam. As can be seen from Figure 1 the fiat mesh screen 40. is supported" closely adjacent to the evaporated photoconductive-rnaterial 18 and, when a potential is appliedto mesh screen 40, the mesh screen functions as a grid electrode, A screen having a minimum of approximately SOO Wir esper inch, when used as in mesh 40, does not show the televised picture. the-evaporator shield 34-by means of a lead-in 52.

Referring now to Figure 3 there is shown an embodiment 'of this invention for a pickup tube of'the type described'above, for use when it is desired to replace the evaporator shield withconventional electrodes. The assembly comprises an evaporator crucible 48" which is supportedwithin' envelope -11 by support rods, or leadins 46'. The evaporator-shield GGCOD'IPIlSCS G. hollow tubular-ametallic member 62 ha-ving both ends open. Contiguous-- with the inner surface 01:" the tubular cylinder. 62 is ad'nesh screen-64. The mesh screen164"extends beyond 'a'ntopenendof' cylinder 62fto a point closely' adjacente to a the transparent signal electrode 16'.

lathe-embodiment ofthe invention shown in Figure .3

the mesh screen 64,-which is mesh screen of 'apprtnrimately 30"x30wi-res per-inch, extends. within the tub'ular member= 62 and is connectedthereto by" anywellknown. means such as by spot welding. In this embodimentthe evaporator shield 60 is preferablyremoved' from the envelope 11! when the operation is completed and conventional elect-rodes inserted within envelope 11".

Therefore, a fiat mesh screen across the open endof shield .60, similar to that shown in Figure 1, is optional. Here again thewcylinder 62 may be a nickel-chromium alloywhilethesmeslr64may be stainless steel. During-the evaporationipnocess the evaporator is preferably supported approximately-" A; inch to 2 inchesfrom the transparent conductive coating 16. The distance between the shield-564and the signal plate 16 may be within the- Also,- as wasithe case: in connectionwith the embodiment shown in approximate range of /2 to 3 millimeters.

Figure 1; no particular meshsize-is-requ-ir'ed; and meshes of 30 wires per-inch up to .150 wires per inch have been and bubbling, while alarge opening, i. e. oneof about .4- inch inside diameter; tends to non-uniformity'with theedges tending to be void" of evaporated material.-

As has-= been "described, in bothembodi'rnents of'this invention, the evaporation-of the photoconductive material, e-.- g. antimonytrisulphide, is preferably donein-an inert atmosphere-of-approximately-%l mm. to-- 35mm; of;

mercury; pressure to provide aporous film" 0t photocone ductive materialz Potentials are applied to The purpose of the mesh 64, inside- What is claimed is:

1. An evaporator shield for evaporating photoconductive material comprising a hollow tubular metallic member, a hollow tubular mesh screen having one open end connected to one open end of said member, an annular conductive ring connected to the other open end of said screen, and a flat mesh screen member connected to said ring and closing the aperture therein.

2. An evaporator shield for evaporating photoconductive material in a pickup tube comprising, a hollow tubular metallic member open at one end and having the other end bonded to a hollow mesh screen member, and the other end of said hollow mesh screen member being connected to a fiat mesh member.

3. An evaporator shield for evaporating photosensitive material in a pickup tube comprising a hollow tubular mesh screen member having one end closed by a flat mesh screen.

4. An evaporator assembly comprising an evaporator cup, means to heat said evaporator cup, an evaporator shield including a hollow tubular mesh screen member, means to support said evaporator cup in spaced relation within said evaporator shield, and a flat screen member closing one end of said shield.

5. An evaporator assembly for evaporating material onto a surface comprising, an evaporator crucible and a hollow evaporator shield, means to support said crucible in spaced relation within said shield, said shield being adapted to have a portion thereof positioned a predetermined distance from said surface, said shield compris- 6 ing a tubular mesh member and a substantially flat mesh screen closing said portion of said shield.

6. An evaporator assembly comprising an evaporator crucible, a hollow evaporator shield, means to heat and to support said crucible in spaced relation within said shield, said evaporator shield including a hollow tubular mesh screen portion and a hollow tubular metallic portion, one end of said mesh portion extending into said metallic portion, and a substantially flat mesh screen closing the other end of said mesh portion.

7. An evaporator assembly comprising an evaporator shield, said evaporator shield comprising a hollow tubular member having a plurality of apertures through the Walls thereof, an evaporator, means to heat said evaporator and to support said evaporator in spaced relation within said evaporator shield.

References Cited in the file of this patent UNITED STATES PATENTS 2,103,623 Kott Dec. 28, 1937 2,378,476 Guellich June 19, 1945 2,416,211 Osterberg et a1 Feb. 18, 1947 2,553,289 Alexander et al May 15, 1951 2,572,881 Rothstein Oct. 30, 1951 2,682,479 Johnson June 29, 1954 2,692,574 Anderson Oct. 26, 1954 FOREIGN PATENTS 570,200 Great Britain June 27, 1945 

1. AN EVAPORATOR SHIELD FOR EVAPORATING PHOTOCONDUCTIVE MATERIAL COMPRISING A HOLLOW TUBULAR METALLIC MEMBER, A HOLLOW TUBULAR MESH SCREEN HAVING ONE OPEN END CONNECTED TO ONE OPEN END OF SAID MEMBER, AND ANNULAR CONDUCTIVE RING CONNECTED TO THE OTHER OPEN END OF SAID SCREEN, AND A FLAT MESH SCREEN MEMBER CONNECTED TO SAID RING AND CLOSING THE APERTURE THEREIN. 